1) Field of the Invention
This invention relates to an optical fiber having a superior nonlinearity and an optical signal processor using the optical fiber.
2) Description of the Related Art
In recent years, demand has increased for the optical signal transmission higher in speed and capacity over a longer distance than ever before. This requires a signal processing technique for a high processing speed and long distance transmission of the optical signal. In one of the optical signal processing techniques, the optical signal is converted to an electrical signal, and the converted electrical signal is processed and restored again to the optical signal. The process of converting an optical signal to an electrical signal and converting the electrical signal again to an optical signal, however, is unsuitable for high speed signal processing.
In contrast, a full optical signal processing technique is available in which the optical signal is processed directly. In this processing technique, the optical signal is directly processed as it is without being converted to an electrical signal, and therefore a high-speed optical signal processing is made possible.
The full optical signal processing technique is implemented by a method utilizing the nonlinear optical phenomenon in the optical fiber for transmitting the optical signal or a method utilizing the nonlinear phenomenon in the optical waveguide made of a material having a high nonlinearity. The former method utilizing the nonlinear optical phenomenon in the optical fiber is now closely watched in, view of the fact that the high-speed processing is made possible while at the same time reducing the transmission loss. The nonlinear phenomena occurring in the optical fiber include the four-wave mixing, self-phase modulation, cross-phase modulation, Brillouin scattering, or the like. Of all of these phenomena, the wavelength conversion using the four-wave mixing, the waveform shaping and the pulse compression using the self-phase modulation have already been reported as an optical signal processing technique.
The four-wave mixing is a phenomenon in which the light of a new wavelength is generated according to a specified rule by the nonlinear phenomenon that occurs when the light of two or more wavelengths are input to the optical fiber. The optical signal processing technique described above is intended to utilize the phenomenon of generating the light of a new wavelength in wavelength conversion. The wavelength conversion utilizing the four-wave mixing has the advantage that many signals having various wavelengths can be collectively converted.
By utilizing the self-phase modulation and the cross-phase modulation, on the other hand, the waveform deteriorated in transmission is shaped and the long-distance transmission of the full optical signal processing is realized.
The application of these optical signal processing techniques including the wavelength conversion and the waveform shaping utilizing the nonlinear phenomena such as the four-wave mixing and the self-phase modulation in the optical fiber requires an optical fiber that has a large nonlinear phenomenon, namely that has a high nonlinearity.
The nonlinear constant is one index of the nonlinearity of the optical fiber. The nonlinear constant is expressed by formula (1) below.Nonlinear constant=n2/Aeff  (1)where n2 is the nonlinear refractive index of the optical fiber, and Aeff is the effective area of the optical fiber. The formula (1) indicates that to increase the nonlinear constant of the optical fiber, it is necessary either to increase the nonlinear refractive index n2 or to reduce the effective area Aeff.
This is realized by a method in which the first core located at the central portion of the optical fiber is doped with a great amount of germanium to increase the nonlinear refractive index n2 or a method in which the relative refractive index difference between the first core and the cladding is increased to reduce the Aeff. An increased relative refractive index difference between the first core and the cladding, however, would increase the dispersion slope and the cut-off wavelength is shifted to long wavelength side.
As a means for solving this problem, what is called the W-type refractive index profile is known in which a second core lower in refractive index than the cladding is arranged on the outer periphery of the first core to decrease the dispersion slope and shift the cut-off wavelength to short wavelength side.
This method is realized by the optical fiber proposed, for example, in Japanese Patent Application Laid-Open (JP-A) No. 2002-207136. In this optical fiber employing the W-type refractive index profile, the concentration of germanium doped in the first core is increased to increase the nonlinear refractive index n2 and the relative refractive index difference between the first core and the cladding is increased to reduce the Aeff. This method can realize an optical fiber having a sufficiently short cut-off wavelength.
To make the W-type refractive index profile of the optical fiber, the refractive index of the second core located on the outer periphery of the first core is reduced by, for example, doping fluorine into the second core.
To obtain a satisfactory characteristic of the optical fiber having a high nonlinearity, however, the refractive index of the second core is required to be increased to negative side, which in turn makes it necessary to dope high-concentration fluorine into the second core.
In doping fluorine in a normal pressure environment, however, the refractive index can be reduced at most to about −0.7% in terms of relative refractive index difference with pure silica.
If the refractive index of the second core is to be reduced below this limit, the technique of doping fluorine in a high-pressure environment is required (“pure silica” is defined as pure silica glass doped with no dopant doped to change the refractive index). A very sophisticated technique is required to dope fluorine in a high-pressure environment and poses the problem that the production equipment is complicated and the production yield is deteriorated at the same time.